A primary reason for providing paper with a filler is to improve certain properties of the paper. One important property in this way is the opacity of the paper, i.e. the non-transparency. Certain fillers improve the brightness and/or the whiteness of the paper. One example of such a filler is PCC (precipitated calcium carbonate), i.e. precipitated calcium carbonate. The filler can also improve the surface smoothness of the paper, resulting in improved printability. In addition, the majority of fillers are significantly cheaper per unit weight (kilogram or tonne) than pulp fibres. This is particularly the case in relation to fully bleached chemical pulps. The admixture of filler thus leads to a reduction in paper manufacturing costs. It is worth noting that there is a risk in using filler, and then particularly in large quantities, as the strength of the paper is impaired to a greater or lesser extent in comparison with paper that includes no filler.
The manufacture of paper that contains a filler commences with the production of a thick pulp suspension. This suspension can be produced in different ways. In the case of paper manufacture based on dry pulp in bale form, the pulp is slushed in water, usually white water taken from the long circulation, such as to obtain a thick pulp suspension. In the case of paper manufacture based on pulp in suspension form that is delivered through a conduit to the paper mill from an adjacent pulp mill, the suspension is usually de-watered initially, for instance from a consistency of about 2% to about 15%, so as to obtain a coherent pulp cake. The resultant water, free from pulp fibre, is sent back to the pulp mill through a conduit for renewed use as a vehicle for feeding fresh pulp fibres to the paper mill. The pulp cake obtained in the paper mill is broken-up and mixed with white water from the long circulation, so as to obtain a thick pulp suspension.
The pulp fibres in the form of a thick pulp suspension are normally subjected to a beating process prior to their further advance in the system. If the pulp furnish includes, for instance, two different pulps, these pulps are usually each beaten separately before mixing the two pulp suspensions together.
Relatively large quantities of paper broke are obtained in the following paper manufacturing process. There are several causes why paper broke is obtained. A constantly dominating cause is because the outer edges of the advancing paper web are cut away as a matter of routine. Scrapping is another cause, i.e. the paper produced does not fulfil periodically the quality requirements placed on the paper. A third cause can be that the advancing paper web breaks-off for some reason or other. Such broke paper is normally passed back to the paper manufacturing chain, after having been slushed in white water in broke pulpers. Because this starting material in the form of paper broke contains a filler, the resultant thick pulp suspension will also contain a filler. The amount of paper broke concerned may be as high as 40%, and even higher, which is, in itself, a problem. However, a more difficult problem in this connection is that the amount of paper broke normally varies with time. This means that the filler content of the incoming thick pulp suspension will also vary with time.
The thick pulp suspension is diluted with white water batch-wise on its way to the short circulation and to the head box. One or more paper chemicals can be delivered to the thick pulp suspension on such suspension diluting occasions. Significant dilution f the pulp suspension with white water takes place at the beginning of the short circulation, for instance in the wire pit, so as to obtain a stock that has a low solids substance content. Fresh filler can be delivered to the pulp suspension at several positions, for instance to the thick pulp suspension or to the stock immediately downstream of the wire pit. The retention agent can be delivered to the pulp suspension at described positions, and also later on in the short circulation, i.e. closer to the head box.
The dominant part of the liquid phase in the pulp suspension is comprised of constantly circulating white water. However, a permanent or temporary white water deficiency can be made up with fresh water.
The majority of fillers are in particle form that have a very small surface area (e.g. a diameter smaller than 10 μm) in relation to the surface area or the size of pulp fibres (having a length of, e.g., 3000 μm and a-width of, e.g., 30 μm). There is a relatively small chance of the filler fastening in the paper web by itself or being spontaneously taken-up by the web. When forming the paper on the wire cloth, practically all pulp fibres will fasten on the cloth and form a bed or network thereon. The number of holes in the network is determined by many factors, among other things by the type of paper producing process applied precisely in the paper machine and also the weight per unit area or grammage of the paper produced. There is a direct connection between an increase in grammage and an increase in the thickness of the pulp fibre bed. It is natural that an increase in pulp fibres bed thickness will result in an increase in the amount of filler that is taken up. However, the spontaneous adsorption or retention of filler is insufficient to provide the desired content of filler in the paper. It is therefore necessary to add one or more substances or chemicals that assist in incorporating filler in the pulp fibres bed and therewith in the wet paper web as it leaves the wire and, e.g. is fed into a press section of the paper machine. The wet paper web is transferred to an endless felt in conjunction therewith. This substance or chemical is designated a retention agent. The use of a retention agent results in comparatively more filler remaining in and accompanying the paper web, and comparatively less filler slipping through the pulp fibres bed and down through the wire cloth and into the wire tray together with the drainage water or white water. Despite the use of a retention agent, and then even in large amounts, only a minor part of the filler present in the stock fed into the head box and thereafter spread on the wire cloth will fasten in the paper web, whereas a major part of the filler will accompany the drainage water as it passes through the paper web and the underlying wire cloth. This means that the amount of filler in the white water is still relatively high and is very large when seen in respect of the total amount of filler in the entire system (and then primarily in the large volume of white water that circulates in both the short circulation and the long circulation).
On the basis of the described circumstances, it will readily be seen that it is difficult to control the production of filler-containing paper in a manner such that the final product, i.e. the finished paper, will constantly and persistently contain the desired filler content or filler consistency, for instance expressed in a given percentage value. The buyer and the user of the paper are interested in that the quality of the paper being always the same, and it is important in this respect that the filler content of the paper is always the intended content and that this filler content is achieved constantly from batch to batch.
In order to enable the manufacture of paper to be controlled in the above-described respect, there have long been used measuring operations that are carried out with the aid of a certain type of measuring apparatus. One of these measuring operations involves determining the filler content of the advancing paper web, normally at the end of the paper machine, by means of a non-destructive measuring process, said filler content sometimes being referred to as the ash content. Another measuring operation involves determining the filler concentration of the white water either in the short circulation or in direct connection therewith. Alternatively, the filler concentration is determined together with the low quantity of pulp fibres that are already present in the white water (total concentration). The two measuring processes are normally carried out intermittently, at intervals ranging from, e.g., only a few seconds to, e.g., thirty seconds between respective measuring occasions.
In conventional control technology, both the addition of retention agent and the addition of filler are varied in the course of making the additions. The amount of retention agent added is based on precisely the amount of filler measured in the white water, and the amount of filler added is based on the measured content of filler in the paper web. It has been found that this control philosophy leads to a relatively significant variation in the filler content of the finished paper. Because the filler content of an accepted paper is only allowed to vary within a narrow range, the paper that must be scrapped becomes much too excessive. Moreover, as a result of this control philosophy, the switch from one filler content to another in the paper, for instance from 15 to 19 percent or vice versa, becomes unnecessarily extended time-wise and therewith necessitates unnecessarily the scrapping of much of the paper. The earlier described problem caused by varying amounts of filler in the incoming thick pulp suspension is not overcome completely by the described control philosophy. The attempt to correct a newly measured excessively low amount of filler in the finished paper with an increased addition of filler to the thick pulp suspension for instance, or to the stock is doomed to failure to some extent, since the total amount of liquid, chiefly white water, in the system is, as a whole, very large, meaning that the amount of circulating filler is also large and also meaning that an instantaneous increase in the amount of filler added to the system is unable to become quickly effective in respect of an increase of the filler concentration in the circulating liquid system, which, in turn, would result in a higher quantity of filler fastening in and being retained by the paper web. Such a system is extremely slow to control for these reasons.
Finnish Patent Application 97 4327 and its corresponding International (PCT) Patent Application WO 99/27182 describes, among other things, a method which is alleged to afford advantages in the form of faster and more effective control of the paper properties in the short circulation of the paper machine, in relation to known techniques. By paper properties is meant primarily the filler content of the paper. It would appear that the method concerned is based on the aforedescribed known technique, which has been supplemented with an incompletely explained process in which both the continuous addition of filler and the continuous addition of retention agent are both based on the measured concentration of filler in the white water and the measured filler content or ash content (which is the term used) of the paper.